RFR: Convert old-gen single threaded pretouch to multi-threaded during

[Kim Barrett]

> On Jan 8, 2021, at 8:08 AM, Amit Pawar <amith.pawar at gmail.com> wrote:
> 
> Hi
> 
> I am trying to improve the pre-touch time taken during old-gen resizing.
> Need your suggestions whether following change will be accepted or not.
> 
> What is happening ?
> Every GC thread resizes the old-gen during object promotion if there is no
> enough room for the object. After expanding GC thread will pre-touch the
> pages alone and cant pre-touch in parallel using PretouchTask task as it is
> already executing a GC task. The total GC pause time depends upon resize
> size and number of resizes.
> 
> What is fix?
> Create another WorkGang and then GC thread can execute pre-touch task with
> this new WorkGang to reduce the pre-touch time taken. The code change is
> given below.

I don't think adding a work gang is the right approach here.  The threads in
that new work gang may just end up competing for CPUs with the already
in-progress work gang doing the normal GC work.

A better approach would be to refactor pretouch parallization to allow
threads to join the fray as needed.  Then arrange for the in-progress work
gang threads to join the pretouch if they would otherwise be waiting for it
to complete.

I've recently been looking at the relevant parts of ParallelGC, and it looks
like it shouldn't be too hard to allow threads waiting for expansion to
cooperate in any ongoing pretouch, esp. after some other recent RFEs have
been dealt with. I've filed JDK-8260332 for this. I haven't looked at the G1
side of things yet.